Diversity of CO2 concentrating mechanisms and responses to CO2 concentration in marine and freshwater diatoms ($)

The CO₂-fixing enzyme ribulose bisphosphate carboxylase (Rubisco) works most efficiently at high concentrations of CO₂. Many organisms have evolved CO₂-concentrating mechanisms (CCMs), such as the PEP-carboxylation that occurs upstream of Rubisco in C₄ plants. Diatoms and other eukaryotic algae use a biophysical mechanism involving transporters of CO₂ or HCO₃^– produced by carbonic anhydrase through the reaction CO₂ + H₂O ↔ H₂CO₃ ↔ HCO₃^– + H⁺. Clement et al. investigated CO₂ fixation and CO₂-concentrating mechanisms in fresh-water and marine diatoms acclimated to varying pH and CO₂ levels. Key findings include no indication of C₄-type CCMs in the diatoms they studied, and no clear differences between marine and fresh-water species, in spite of the fact that CO₂-levels are much more variable in fresh water. J Exp. Bot. 10.1093/jxb/erx035